I noticed that this surname was recently added to the Scot-Irish section, to which I have to quibble. While thedennis does make mention of it being Scot-Irish and German, there's no link to the German page, which I believe to be the more important of the two origins.
Speaking as one who has done family tree research (this was my mother's maiden name) on the surname, it primarially seems to be a very regional name originally to Pennsylvania and it branched out from there, originally starting with the German community as the first bearers of the name in my family tree have the following names:
Johannes Simmers
Wilhelm Simmmers
Joh. Georg Simmers
Elisabetha Simmers
Children of:
Jakob & Margaretha
Most likely it's a bastardized version of the far more common German surname: Zimmers.
However there is a possibility that it's somehow related to the English surname of Simmons or Simms, in which it might be a derivative of the given name Simon; akin to how Zimmers and Zimmerman are related to one another in German, but that's pure speculation of how it might have a non-Germanic origin. So far in my family tree research it's only turned up to be a name that starts in Pennsylvania (there seems to be two branches--one that started and stayed in the Chester County region (my branch) until the 20th Century, and another which started in Western PA and moved west into Ohio).
Anyone else have any thoughts?